Introduce MPT support (XLS-33d): (#5143)

Amendment:
- MPTokensV1

New Transactions:
- MPTokenIssuanceCreate
- MPTokenIssuanceDestroy
- MPTokenIssuanceSet
- MPTokenAuthorize

Modified Transactions:
- Payment
- Clawback

New Objects:
- MPTokenIssuance
- MPToken

API updates:
- ledger_entry
- account_objects
- ledger_data

Other:
- Add += and -= operators to ValueProxy

Read full spec: https://github.com/XRPLF/XRPL-Standards/tree/master/XLS-0033d-multi-purpose-tokens

---------
Co-authored-by: Shawn Xie <shawnxie920@gmail.com>
Co-authored-by: Howard Hinnant <howard.hinnant@gmail.com>
Co-authored-by: Ed Hennis <ed@ripple.com>
Co-authored-by: John Freeman <jfreeman08@gmail.com>

src/libxrpl/protocol/STLedgerEntry.cpp

@@ -125,6 +125,10 @@ STLedgerEntry::getJson(JsonOptions options) const
 
     ret[jss::index] = to_string(key_);
 
+    if (getType() == ltMPTOKEN_ISSUANCE)
+        ret[jss::mpt_issuance_id] = to_string(
+            makeMptID(getFieldU32(sfSequence), getAccountID(sfIssuer)));
+
     return ret;
 }